125 lines
3.2 KiB
C
125 lines
3.2 KiB
C
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/*
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* d16verb.c
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*
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* Decoding verbatim-bit blocks
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*/
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#include "decoder.h"
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int NEAR decode_verbatim_block(
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t_decoder_context * context,
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long BufPos,
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int amount_to_decode /* yes, it will equal 32768 */
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)
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{
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ulong match_pos;
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ulong dec_bitbuf;
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byte *dec_input_curpos;
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byte *dec_end_input_pos;
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int match_length;
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int c;
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char dec_bitcount;
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char m;
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MATCH match_info;
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/*
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* Store commonly used variables locally
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*/
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dec_bitcount = context->dec_bitcount;
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dec_bitbuf = context->dec_bitbuf;
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dec_input_curpos = context->dec_input_curpos;
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dec_end_input_pos = context->dec_end_input_pos;
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/*
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* amount_to_decode must never be > 32768
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*
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* As it stands, we do the (amount_to_decode >= 0) check
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* at the bottom of the DO loop, rather than at the top of
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* a WHILE loop, so that we can used a signed int; this way,
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* we decrement it by at least 1 by the time we check against
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* zero.
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*/
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do
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{
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/* decode an item from the main tree */
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DECODE_MAIN_TREE(c);
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if ((c -= NUM_CHARS) < 0)
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{
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/* it's a character */
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/* note: c - 256 == c if c is a byte */
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context->DComp_Token_Literal(context, (int) ((byte) c));
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amount_to_decode--;
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}
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else
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{
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/* get match length header */
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if ((match_length = c & NUM_PRIMARY_LENGTHS) == NUM_PRIMARY_LENGTHS)
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{
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/* get match length footer if necessary */
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DECODE_LEN_TREE_NOEOFCHECK(match_length);
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}
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/* get match position slot */
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m = c >> NL_SHIFT;
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/* read any extra bits for the match position */
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if (m > 2)
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{
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if (m > 3) /* dec_extra_bits[m] != 0 */
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{
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GET_BITS17_NOEOFCHECK(dec_extra_bits[ m ], match_pos);
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match_pos += MP_POS_minus2[m];
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}
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else
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{
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match_pos = MP_POS_minus2[3];
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}
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/*
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* Add match base to "extra bits". Our match base
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* table has 2 subtracted from all the elements.
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*
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* This is because encoded positions 0,1,2 denote
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* repeated offsets. Encoded position 3 denotes
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* a match 1 character away, 4 encodes 2 away, etc.
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* Hence the subtraction of 2, which has been
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* incorporated into the table.
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*/
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/* update LRU repeated offset list */
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context->dec_last_matchpos_offset[2] = context->dec_last_matchpos_offset[1];
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context->dec_last_matchpos_offset[1] = context->dec_last_matchpos_offset[0];
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context->dec_last_matchpos_offset[0] = match_pos;
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}
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else
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{
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/* positions 0, 1, 2 denote repeated offsets */
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match_pos = context->dec_last_matchpos_offset[m];
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if (m)
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{
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context->dec_last_matchpos_offset[m] = context->dec_last_matchpos_offset[0];
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context->dec_last_matchpos_offset[0] = match_pos;
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}
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}
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/* match lengths range from 2...257 */
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match_length += MIN_MATCH;
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match_info.Len = match_length;
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match_info.Dist = match_pos;
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context->DComp_Token_Match(context, match_info);
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amount_to_decode -= match_length;
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}
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} while (amount_to_decode > 0);
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context->dec_bitcount = dec_bitcount;
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context->dec_bitbuf = dec_bitbuf;
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context->dec_input_curpos = dec_input_curpos;
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return 0;
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}
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